In the end of Act 1 Hamlet has just discovered that his fathered has been murdered by Claudius. Unfortunately, it is a ghost that delivered this fateful news and Hamlet can't just jump into a fight with Claudius and kill him in an act of revenge for this father. He must prove that the ghost is telling the truth and is not a ghost merely in the guise of his father but is actually the devil tempting him to do evil and damn his eternal soul. To that end, Hamlet's plan is to "put an antic disposition on." He goes on to specifically explain how he might start to act and what he might say in his 'crazy act' so as to not alarm his friend, Horatio. He says,

That you, at such times seeing me, never shall,

With arms encumb'red thus, or this head-shake,

Or by pronouncing of some doubtful phrase,

As "well, well, we know," or "We could, an if we would,"

Or "If we list to speak," or "There be, an if they might,"

Or such ambiguous giving out,"

By explaining in a rational way what his plan is he is clearly telling us that he just pretending to be mad.